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two story structural integrity questions  RSS feed

 
Gregory Kitchens
Posts: 2
Location: East Texas
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hello everyone! I'm new around here!

I'm no stranger to forums though, and I've done quite a bit of surfing in my search for answers to my questions here on this forum, on Google, Youtube, and many other forums.

First some information. We are building a two story pole and cob style house. The framing is up, as far as the skeleton goes. Its a square structure with telephone poles set for all four corners, with vertical cedar trees between. A good extra thick cedar stands in the middle and the framework for the second floor is up as well (with a few sheets of plywood nailed down to walk up there) the second story floor is cedar and ash trees laid out every two feet i think. We've built a model of the bottom floor to get a good idea of cob mixture and durability, and we're currently building a rocket stove technology oven to get some experience with the mixture and shaping.

Now for the questions. We've found sources for free or cheap pallets, and were wondering a few things before proceeding to build the walls out of pallets and just cob each side of the pallets with straw being the insulation.

1) I've read here on permies that cob is much stronger by its self than with wood inside it, is this true?

2) If that is true, then would it be better to build the two story house with just cob rather than pallets?

3) If it is better to build the walls out of solid cob, how thick should the walls be in order to support the weight of the second story?

4) Any advice for this project would be greatly appreciated.

5) (i forgot) If it is ok to go with pallets, is there a better, no cost way to insulate than to simply stuff the pallets with Straw?

I will update this post with pictures as soon as the rain stops. for now i can only say that it is AWESOME!

EDIT: Not to nitpick, but for anyone that replies, could you provide a source for your information, or some further reading? I'm sure anyone can understand that we dont want to build a house that could colapse on us simply because a random stranger says its ok. Thanks for your help!
 
Joseph Davenport
Posts: 8
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So I have no actual building experience, but I have been to a cob workshop and rocket stove workshop as well as reading a good deal on the subject so I'll give you my knowledge on the first three questions...

1). Cob shrinks when it dries and this can create draft/structural issues if you put it inside the cob. You can refer to Becky Bee's Cob Builder's Handbook for this one (pole structure) I know for sure. I don't have the book handy but there is no issue with having poles to support the roof or house frame but you want to "cup" them with the cob, inside or outside the building, as opposed to inside the wall for structural reasons.

2) I don't believe it makes a difference what you build the second story out of, but cob would be easier. You don't have to worry about joining two different building mediums with deadmen or any other system if you build out of just cob because it is monolithic. No source for this one, just knowledge garnered from studying putting different materials together. The cob is more than sturdy enough to build two stories with. I've stayed in a couple two story buildings built with just cob.

3) It is suggested that you taper cob walls. The formula I got from the builder I worked with was taper 2 inches for every three feet of wall height. Walls should be about 9 inches at the top so measure the height of the wall and it's a simple math equation. I believe this info is in Becky Bee's book as well, and the Hand Sculpted House if I remember correctly.

Hope that helps a bit, and hopefully we can get some more experienced responses. I'm really interested in this as well! Good luck with the build!
 
Gregory Kitchens
Posts: 2
Location: East Texas
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Joseph Davenport wrote:So I have no actual building experience, but I have been to a cob workshop and rocket stove workshop as well as reading a good deal on the subject so I'll give you my knowledge on the first three questions...

1). Cob shrinks when it dries and this can create draft/structural issues if you put it inside the cob. You can refer to Becky Bee's Cob Builder's Handbook for this one (pole structure) I know for sure. I don't have the book handy but there is no issue with having poles to support the roof or house frame but you want to "cup" them with the cob, inside or outside the building, as opposed to inside the wall for structural reasons...!


Thanks for the detailed information! let me rephrase some of my own information. the only wood in the structure are the support poles, and the trees that are the floor joists for the second story. We were only wondering about using pallets because we thought it might save a BIT of time and it would ultimatly mean a BIT less cob. but I suppose shortcuts that cost structual strength, probably isnt the way to go. The roof will be metal roofing (R panel) so I don't believe roof weight will be much of an issue, we were mostly concerned about structural integridy of the second floor, being that we are doing a pole and cob style house. Thank you very much! any more info anyone else can provide would be greatly appreciated!
 
Gord Baird
Posts: 6
Location: Victoria BC
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Hi Gregory,
I built and live in a loadbearing two story cob house in Victoria BC (west coast) - high seismic area. It is code approved and permitted (first legal 2 storey seismacally engineered load bearing cob home in North America). I've also helped four other families do the same. There is no one way to build a cob house... but I would recommend the following.

Wall thickness is determined based on the compressive load it has to carry as well as the thermal performance you need to gain. (For thermal performance we have done some intense research found here under the "technical science report" http://ecosenseliving.wordpress.com/research/ ). Cob is stronger as a monolithic unit then with things embedded in it. If you are building in a seismic area, then every 2 feet an engineer will want to see a horizontal layer of some form of grid-like material (geotextile or welded wire fencing)... if this is the case then you can build vertically... if you don't have to use this horizontal layer then the sloped walls are best.

As for detailing air gaps - if you detail the building properly, then it can be an airtight building. In BC, the Home Protection Office provides classes and training for builders and building officials... they classify cob as the most air tight building system - but if your connections at poles and roofsare not detailed well then its not. A word of warning about doing a load bearing structure with poles and infilling with cob... Gernot Minke has a section in his book Building with Earth where he warns about using two materials with different flexing properties adjacent to each other. Wood flexes under loads and wind shear whil cob does not. Eventually at these connections you will see failure. The route around this is to never have your cob come into contact with the supporting posts, something we are seeing adhered to more and more. Also some great earthen standards out there including the ASTM2392 which discusses some do's and don'ts.

Good luck and enjoy... it will be the healthiest building you will ever live in (in MHO).
 
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